Advances in science and engineering in the past half-century have led to the widespread use of nuclear power to provide a significant portion of the world's energy needs. Commercial nuclear reactors use the principle of nuclear fission to generate the energy that is eventually converted to electricity, usually by steam cycles which incorporate turbine generators. Nuclear fission is a process in which the nuclei of isotopes of certain heavy elements, such as uranium or plutonium, are bombarded with free neutrons. Upon absorption of the free neutrons, the nuclei of the isotopes will, under certain conditions, split, or "fission." There is a resulting release of energy, most of which is in the form of kinetic energy of fission products. This energy is eventually converted into electricity.
The general method of reactor control used in industry is to control the number of free neutrons available for the fission process in the core of the reactor. Such neutron population control may be achieved by several methods or combinations thereof. For example, some reactors are designed such that only neutrons of a specific energy level are capable of causing the nuclear fuel to fission. In these reactors it is often the case that as the fission rate increases, neutrons of the appropriate energy level "leak" out of the core at a faster rate. This decreases the number of fissions in the core. Thus, the number of useful neutrons available for fission decreases in proportion to the fission rate increase. This design feature provides a kind of negative feedback which tends to force the nuclear processes within the core toward stability and steady state.
Another method of neutron control is to intersperse, within the nuclear fuel, a neutron absorber, or "poison." A simple poison-loading concept might be to distribute spherical granules of poison throughout the core. Early in core life, when core reactivity may be highest, the effective surface area of the poison spheres is at a maximum, exposing the greatest number of neutron-absorbing poison isotopes to the neutron flux. As the core ages, and both nuclear fuel and poison are depleted, the effective surface area of the poison spheres is reduced proportionately. This exposes fewer neutron-absorbing poison isotopes to the neutron flux and thereby compensates for the depletion of fuel and corresponding reduction in core reactivity.
A third and widespread method of neutron population control is incorporation of "control rods" into reactor design. Control rods are movable assemblies that contain neutron-absorbing isotopes. Because reactor cores are generally designed so that small movements of the control rods by the reactor operator can cause significant and immediate changes in core reactivity, this method is incorporated in most reactors as a primary means for human-interactive reactor control.
The effectiveness of control rods depends upon direct insertion of the rods into the reactor core. Thus, the structural elements of the rods themselves must be capable of providing support while being subjected to the extremely high temperatures of the core interior. In addition, many reactor designs require some control rod flexibility. This is so because the channels into which the control rods are inserted are long and extend from structures high above the core, down through the head of the pressure vessel which houses the core, and into the core itself. Design requirements also often require these long channels to be constructed with very little excess clearance between the rods and the channels. Problems then can arise when the normal settling and shifting inherent in any large structure causes relative misalignment in the control rod channels. Foundation shifting due to earth movement may also cause such channel misalignment Even though any such misalignment will likely be relatively minor, perhaps as small as a few microns, it may easily be enough to prevent the unimpeded movement of a rigid control rod within its channel.
Thus, the control rod structure must be flexible to ensure effective reciprocation within a potentially misaligned channel It must also be capable of providing sufficient strength for support over a wide range of extreme temperatures. One way to achieve such flexibility is to construct the control rods as a series of interconnected segments which are flexible at their respective joints.
Conventional flexible rod designs are composed of materials that provide strength which is adequate throughout much, but not all, of the temperature range of the core interior. For example, many control rod materials are incapable of withstanding the intense temperatures at the core interior generated during a conduction cooldown event in a high temperature gas-cooled reactor design, which may be as high as 2500.degree. F. This necessitates removing or "locking out" interior rods during reactor operation as a precautionary measure. As a result, these rods cannot be used during reactor operation, and cannot be considered as components of "reactor safety equipment". This requires the added expense of modified reactor design and additional systems that can fulfill federally promulgated reactor safety guidelines.
Newer artificial graphites and composite materials, such as graphite or carbon-carbon, can withstand the intense temperatures at the core interior during conduction cooldown events. Conventional structures made of these composite materials, however, are structurally weaker at segment joint connections and, hence, are unsuitable for use as strength members.
It is therefore an object of the present invention to provide for unimpeded insertion of control rods into potentially misaligned control rod channels. It is a further object of the present invention to provide for control rod operation across the entire nuclear core in extreme temperature ranges, such as those found in certain gas-cooled reactors during conduction cooldown events. It is a further object of the present invention to provide a control rod assembly which is durable, reliable, and cost-effective in its manufacture and use.